netdev - Re: [PATCH net] netlink: do not enter direct reclaim from netlink

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Message-ID: <20161005234425.GA87410@ast-mbp.thefacebook.com>
Date:   Wed, 5 Oct 2016 16:44:26 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Greg Thelen <gthelen@...gle.com>, Chris Mason <clm@...com>,
        kernel-team@...com
Subject: Re: [PATCH net] netlink: do not enter direct reclaim from
 netlink_dump()

On Thu, Oct 06, 2016 at 08:35:21AM +0900, Eric Dumazet wrote:
> On Wed, 2016-10-05 at 15:24 -0700, Alexei Starovoitov wrote:
> > On Thu, Oct 06, 2016 at 04:13:18AM +0900, Eric Dumazet wrote:
> > > 
> > > While we are at it, since we do an order-3 allocation, allow to use
> > > all the allocated bytes instead of 16384 to reduce syscalls during
> > > large dumps.
> > > 
> > > iproute2 already uses 32KB recvmsg() buffer sizes.
> > ....
> > > diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
> > > index 627f898c05b96552318a881ce995ccc3342e1576..62bea4591054820eb516ef016214ee23fe89b6e9 100644
> > > --- a/net/netlink/af_netlink.c
> > > +++ b/net/netlink/af_netlink.c
> > > @@ -1832,7 +1832,7 @@ static int netlink_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
> > >  	/* Record the max length of recvmsg() calls for future allocations */
> > >  	nlk->max_recvmsg_len = max(nlk->max_recvmsg_len, len);
> > >  	nlk->max_recvmsg_len = min_t(size_t, nlk->max_recvmsg_len,
> > > -				     16384);
> > > +				     SKB_WITH_OVERHEAD(32768));
> > 
> > sure, it won't stress it more than what it is today, but why increase it?
> > iproute2 increased the buffer form 16k to 32k due to 'msg_trunc' which
> > I think was due to this issue. If we go with SKB_WITH_OVERHEAD(16384)
> > we can go back to 16k in iproute2 as well.
> 
> Wow, if really iproute2 tool would have increased the buffer to work
> around a bug in the kernel, we should be worried.
> 
> Hopefully the issue was fixed for good in the kernel ?
> 
> commit db65a3aaf29ecce2e34271d52e8d2336b97bd9fe
> ("netlink: Trim skb to alloc size to avoid MSG_TRUNC")

I would think so too, but
iproute2 'fix' 72b365e8e0fd ("libnetlink: Double the dump buffer size")
was on Mar 4, 2016
whereas above 'netlink: trim' fix is on Oct 15, 2015